Pigments of the formula X.sub.a Y.sub.b (Z(L1).sub.n (L2).sub.m).sub.c, where X is an alkaline earth metal, Y is a transition metal or heavy metal, Z is a transition metal or heavy metal, L1 and L2 are ligands, which can be mono-dentate ligands, such as, for example, cyano or carbonyl moieties, or bi-dentate ligands, such as, for example, nitroso-beta-naphthol as in Pigment Green B, a equals 1 or 0, b is an integer from 1-6, c is an integer from 1-6 and n and m equal an integer from 0 to 6, are ionic in character, and thus highly polar. As a result, dispersing pigments of the above formula into non-polar solvent media is difficult, expensive, and time-consuming. One member of this pigment family is milori blue, or "iron blue," which is a class of blue pigments composed of either iron(III)hexacyanoferrate II or iron(II)hexacyanoferrate III. These pigments are in essence iron complexes composed of iron ions and cyano moieties. Several methods have been disclosed in the art for creating milori blue pigment dispersions, but none are entirely satisfactory.
For example, U.S. Pat. No. 4,277,288 discloses a method of dispersing milori blue pigment by milling and sieving the pigment particles, which is both expensive, and time consuming.
In addition to the aforementioned use of mechanical force, the use of anionic surfactants, in situ polymerization, and encapsulation have been proposed to prepare stable, homogenous dispersions of milori blue. Specifically, JP 51082317 discloses the use of alkylphosphonic acid, while JP 6145025 discloses the use of soy lecithin. In situ polymerization is proposed in U.S. Pat. No. 5,420,317, and encapsulation of the pigment particles by cationic polymers is disclosed in FR 2,673,838. In each of these cases, however, relatively expensive ingredients are required, and milling is still required to obtain a stable, homogenous dispersion.
U.S. Pat. Nos. 3,398,113 and 4,160,760, disclose the preparation of a dispersion of milori blue, using an emulsion-type, two phase system, in which, if organic media is used, water removal is difficult and expensive.
Finally, U.S. Pat. No. 4,545,821 discloses preparing a dispersion of milori blue by pre-depositing the milori blue pigment particles onto mica flakes. This process is simply not economical on an industrial scale.
Thus, a need exists for a fast and economical way to prepare a stable milori blue pigment dispersion. A need also exists for a method of preparing milori blue pigment particles, so those particles can be easily dispersed in a wide variety of solvents. These needs are met by the method of the present invention.